1. Field of the Invention
The present invention relates to an apparatus and a method for retransmitting data in a wireless communication system. More particularly, the present invention relates to an apparatus and a method for selectively retransmitting data according to a channel state in the wireless communication system.
2. Description of the Related Art
As a wireless communication system transmits data using radio resources, the transmitted data may be subject to an error due to a channel state. To enhance reliability of the data transmission and a reception performance, the wireless communication system retransmits the corrupted data using a retransmission scheme (e.g., Automatic Retransmission reQuest (ARQ)).
To perform the ARQ in the wireless communication system, a receiver needs to send information relating to error in the data received from a transmitter, to the transmitter. For instance, the receiver of the wireless communication system determines if there is an error in the data received from the transmitter. When the data has no error, the receiver sends an Acknowledgement (ACK) signal to the transmitter. When the data is corrupt, the receiver sends a Negative ACK (NACK) signal to the transmitter.
When receiving the ACK signal, the transmitter may transmit new data to the receiver. When receiving the NACK signal, the transmitter retransmits the original data associated with the NACK signal to the receiver.
As such, the wireless communication system can enhance the reliability of the data delivery and the reception performance using the ARQ scheme. However, when the wireless communication system adopts the ARQ scheme according to the channel state, the reception performance may be degraded, compared to the transmission without ARQ. For example, when the wireless communication system uses or does not use the ARQ scheme based on the channel condition, the reception performance changes as shown in FIGS. 1A and 1B.
FIGS. 1A and 1B are graphs illustrating resulting changes in reception performance when using a conventional data retransmission in a wireless communication system.
FIG. 1A illustrates a resulting change in reception performance based on a packet loss rate on the assumption that there is no ARQ feedback loss in the wireless communication system, and FIG. 1B illustrates a resulting change in reception performance based on an ARQ feedback loss rate on the assumption that there is no packet loss in the wireless communication system. Herein, the packet loss rate indicates a probability of error in the data transmitted from the transmitter to the receiver, and the ARQ feedback loss rate indicates a probability of error in the ACK/NACK signal fed back from the receiver to the transmitter.
When the packet loss rate is low in FIG. 1A, the reception performance 100 with the ARQ scheme is better than the reception performance 110 without the ARQ scheme. Yet, as the packet loss rate passes beyond a certain level, the reception performance 100 with the ARQ scheme falls below the reception performance 110 without the ARQ scheme. That is, using the ARQ scheme in the wireless communication system, the reception performance is likely to degrade because the number of data retransmissions increases when the packet loss rate is beyond a certain level and the reception standby time for the NACK signal at the receiver increases.
Without the ARQ scheme in FIG. 1B, there is not a significant change of the reception performance 130 based on the ARQ feedback loss. However, when the ARQ scheme is employed and the ARQ feedback loss occurs, the transmitter does not accurately know ACK/NACK information of the data sent to the receiver. In this case, the transmitter unnecessarily retransmits the data or the receiver waits longer to receive the data for the NACK signal because the transmitter does not transmit data for the NACK signal. As a result, the reception performance 120 is degraded.
As discussed above, the wireless communication system can raise the reliability of data delivery and reception performance by retransmitting the corrupted data using the retransmission scheme. However, depending on the state of the channel environment in the wireless communication system, using the retransmission scheme can degrade performance as compared to the case on which the ARQ scheme is not used.